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You can buy a large roll of duct tape and use that to seal openings. You may have to change it frequently - it can't be used under water, but is pretty good at sealing tanks. I assume you already have a cover. If not, you'll have to use a piece of acrylic and cut out holes for the skimmer and any other piece of equipment, then seal around that. You have to allow for some ventilation. I taped plastic mesh over some holes to provide for that.